[A role of periodontal afferents in the control of jaw-closing muscle activities].

The purpose of the present study was to analyse the quantitative relationship between the property of food and the jaw-closing muscle activities as well as how the periodontal afferents participated to establish such a relationship. The following experiments were performed in the urethane anesthetized rabbits: 1) EMG activities of the masseter muscle during the cortically-induced rhythmical jaw movements (CRJMs) before and during chewing rubber strips with different hardness were compared each other. 2) Effects of combined sections of the maxillary and the inferior alveolar nerves on the masseteric EMG responses during chewing a strip were analysed. 3) Effects of the transection of the trigeminal spinal tract on the above masseteric responses were also analysed. The results were as follows: 1) The masseteric activities during the CRJMs differed depending on the site of the cortical stimulation. According to the level of the masseteric activity, CRJMs were divided into two types; one was the opening-dominant jaw movement (ODJM) with small masseteric activity, and the other was the closing-dominant jaw movement (CDJM) with large masseteric activity. In any CRJMs, the masseteric activity became larger during the insertion of a strip, but the increment was more prominent for the ODJM. The rate of increment of the masseteric activity became larger with the hardness of the strip. 2) After the periodontal deafferentation, the rate of increment of the masseteric activity during chewing a strip was remarkably reduced, especially in the ODJM. On the contrary, deafferentation was less effective to that in the CDJM. 3) The effects of the transection of the trigeminal spinal tract were similar to those of the trigeminal deafferentation. From these results, the following conclusions were obtained: 1) The masseteric activity was facilitated with the hardness of the strip. Furthermore, the rate of increment of the masseteric activity during chewing a strip differed depending on the masseteric activity induced before the strip insertion. 2) Periodontal afferents played an important role in the above facilitatory effect, especially when the masseteric activity was small. 3) The contribution of the periodontal afferents which descended through the trigeminal spinal tract was suggested to be involved in the facilitatory response of the masseter muscle during chewing a strip.